Centrifugal pumps are commonly used to move fluid from one location to another, for example to remove fuel from an aircraft fuel tank and provide the fuel to remote equipment, e.g., to a turbine engine, or to move the fuel from tank to tank. Centrifugal pumps typically have a motor and an impeller enclosed within a housing. The motor rotates the impeller, which in turn draws fluid through an inlet opening in the housing and discharges the fluid (typically under pressure) through an outlet opening. The impeller has an axially-extending stem coupled to the drive shaft of the motor. Radially extending helical blades are formed integrally with the stem of the impeller and are enclosed by an axially-extending cylindrical shroud or sleeve. The stem and helical blades rotate within the shroud to draw the fluid into a volute chamber in the housing. The volute chamber converts the kinetic energy imparted to the fuel by the impeller into pressure and discharges the fluid through the outlet in the housing. Centrifugal pumps are available from a wide variety of manufacturers, including the assignee of the present invention. Exemplary centrifugal pumps are also shown in Carter, U.S. Pat. Nos. 3,652,186; Ridland, 2,846,952; Kalashnikov, 4,275,988; Davis et al, 4,142,839; and Bell, 3,038,410.
Certain centrifugal pumps mounted within fuel tanks are known as "wet pumps." These explosion-proof pumps are typically orientated vertically within the tank--with the motor being located above the impeller in the direction of fuel flow. A certain minimum floor clearance must be maintained between the blades of the impeller and the bottom wall or floor of the tank to provide efficient pumping of fluid. If the impeller is too close to the bottom wall of the tank, "inlet choking" of the impeller can occur and the impeller will not draw fuel efficiently. This floor clearance is evident in the impellers illustrated in the patent references cited above. The preferred floor clearance for a centrifugal pump is easily determined by those of ordinary skill in the art knowing the structural characteristics of the centrifugal pump, and as a rule of thumb, is about 50% of the outer diameter of the impeller.
However, any set-off between the impeller and the floor of the tank necessarily results in unusable fuel. That is, the impeller can only pump fuel down to the bottom of the impeller blades. Any fuel below this level cannot be reached by the impeller. If the fuel tank is mounted within an aircraft, the unusable fuel adds to the total weight of the aircraft. This additional weight can be substantial. For example, in a typical airplane fuel tank of 30 square feet in width, every unusable inch of fuel increases the weight of the airplane by approximately 120 pounds. When this amount is multiplied by two or more fuel tanks in an airplane, the weight factor of this unusable fuel becomes an important consideration in airplane design, particularly when fuel economy and flight range are important factors.
One known method for addressing this problem has been to mount the pump in a recess in the fuel tank. The fuel flows into the recess as the pump is drawing the fuel out of the tank, thereby resulting in removing fuel to a lower level. Ridland, U.S. Pat. No. 2,846,952, shows such a configuration. However, this method is not without drawbacks. For example, if the fuel tanks are already installed in an aircraft, the tanks must be removed and modified or replaced with tanks having a recess, which increases the cost of the aircraft as well as removes the aircraft from service for an extended time. Further, some fuel tanks are formed from flexible bladders enclosed by a rigid frame or shell. The bladders do not lend themselves to any particular configuration and hence make it difficult to form a useable recess.
In any case, it is believed that there is a demand in the industry for a centrifugal pump which can operate with existing tank configurations, does not require costly modification or downtime of the aircraft, and maximizes the amount of usable fuel in a fuel tank.